Control rod for nuclear reactor



Feb. 11, 1964 A. M. HARRIS ETAL.

CONTROL ROD FOR NUCLEAR REACTOR 2 Sheets-Sheet 1 Filed Oct. 31, 1960 Feb. 11, 1964 A.M.HARR1s ETAL 3,121,045

' coNTRox. non FOR NUCLEAR REAoToR 2 Sheets-Sheet 2 Filed Oct. 31, 1960 United States Patent O 65,258 6 Claims. (CL

The present invention relates to a nuclear reactor and 1s more particularly concerned with an improved construction and arrangement for control rods and the control rod drive mechanism for nuclear reactors.

In a nuclear reactor, there is provided a plurality of control rods which include neutron absorbing or poison material and which are disposed for movement relative to a position fully within the reactor core in order to thereby regulate the rate of nuclear reaction within the core. Means are provided for moving the control rods relative to the reactor core, and at least some of the control rods include means for substantially instantaneously driving the control rod into the reactor core in order to scrarn the reactor when an emergency situation exists.

It is the primary object of the present invention to provide a novel and improved form of control rod assembly, which is particularly constructed and adapted to accommodate distortions, expansion and other movements and imperfections present or taking place in the reactor core. A further object ofthe invention is to provide an improved control rod whichvis detachably mounted in the reactor vessel, and which is constructed so as to permit angular deviation of :the control rod with respect to the vertical axis of the reactor vessel. Still another object of the invention is to provide a control rod of the type described, wherein means is provided for internally cooling the control rod in a manner ailording a parallel iiow of coolant through the control rod. It is also an object of this invention to provide a novel and improved drive mechanism for control rods utilizing pressure fluid tor both regulating movement and scram movement ot the control rod. Still another object of the invention is to provide an improved control rod drive mechanism, including a hydraulic motor which is operated at one speed for regulating movement of the control rod and which is operable at a substantially greater speed for scramming the control rod in an emergency situation.

Other objects and advantages will become apparent from the following description ot the selected embodiment illustrated in the accompanying drawings, wherein:

FiGURE l is a cross-sectional view of the control with portions broken away and in section;

FIGURE 2 is a sectional view taken 2-2 in FlGURE l; and

FIGURE 3 is a cross-sectional view drive means, with the hydraulic circuit means being shown schematically.

The illustrated embodiment includes the control rod assembly, shown in FIGURE 1 and indicated by the numeral 10, and a drive assembly 12 for the control rod, seen in FlGURE 3, which are particularly adapted for use with nuclear reactors of the gas cooled type wherein the reactor vessel is disposed vertically and the control rods are positioned for movement into and out of the reactor core from a position below the core. However, it will become apparent as the description progresses, that various features of the structure described herein may also be used -to advantage in connection with other types of nuclear reactors.

ln the illustrated embodiment, the nuclear Vreactor includes a pressure vessel having a bottom wall 14 (FIG- URE 3), and intermediate the bottom wall 14 and the reactor core (not shown) containing the fuel elements there rod,

along the line of the control rod for the fluid drive ice is a suitable thermal shield (not shown) and a bottom grid plate i8 which provides a support for the control rod guide tubes. Since the conguration ot the pressure vessel and the detailed construction of the reactor core are not essential elements of the present invention it is believed that detailed descriptions of these structures are unnecessary to a proper understanding ot the present invention.

With reference particularly to FIGURE 1, it will be seen that the control rod assembly lli includes an outer sleeve or guide tube 2li, which is adapted to extend from the bottom grid plate i8 to a position adjacent the top of the reactor core. The bottom portion of the guide tube 2t) has a detachable connection with a supporting bearing or joint 22 carried by the grid plate. As seen in FIG- URE l, the guide tube supporting joint 22 is in the nature of a spherical bearing 2d secured to the lower end of the guide tube Ztl, through a bayonet connection including ins 26, and the bearing is carried in a mounting 2S which is ixed within the grid plate, to thereby alford angular movement of lthe guide tube relative to a vertical axis. The upper portion of the bearing mounting 2S includes a circular opening Sii providing suilicient tolerance to accommodate a predetermined amount of tilting movement of the guide tube 2d. The amount of tilting is determined by the engagement ot a series ot lugs 32, or the like, on the bearing mounting 2% with an interrupted groove 3ft on the outer surface of the spherical bearing 24. This mounting feature of the guide tube is of importance, particularly in that it permits the guide tube 26, and the control rod contained therein, to yield somewhat to the lateral pressures exerted on the control rod assembly lb by the adjacent fuel elements which are ordinarily positioned within the reactor core in surrounding relation to the control rod.

The control rod within the guide tube 2t) includes an upper reflector portion 36 ot graphite or the like, which has a reduced neck portion 38 projecting beyond the upper end of the guide tube 2t), in order to permit engagement thereof by a grappler mechanism (not shown) for removal ot the control rod assembly lil Jfrom the reactor core. The top reilector portion 36 is suitably connected, as by a screw threaded coupling, indicated at 40, with the upper one of a plurality of control rod sections 42. The control rod sections i2 are disposed throughout the length of the guide tube 29, and each of these sections includes an internal support rod 44, preferably of metal, which includes a pair of enlarged cap portions 46 and 48 at opposite ends thereof for supporting therebetween a neutron absorbing or poison material 50, such as gra phite with boron carbide, or the like. Additionally, the poison section 59 may be further supported in position by one or more transverse pins 5l, which extend through the section 5@ and the support rod 44. This poison section 5i) is cylindrical in coniiguration and is disposed with the inner wall thereof disposed in spaced relation to the support rod 44, and the outer wall of the poison material Si) is spaced from the inner wall of the guide tube 2t). This arrangement of the neutron absorbing material provides ior parallel passage of a gas coolant through the interior ot the control rod in a manner to be later described in detail.

The adjacent ends of adjoining pairs of the control rod sections 42 are joined by a knuckle joint 52 or the like, which affords relative movement of the rod sections about a pair of horizontal axes which are disposed generally at right angles to each other. More particularly, the illustrated form of the joint 52 includes a yoke-like member 54 which is xed to the lower end of one support ro dil and which presents a generally spherical bearing surface engaging the inner wall of guide tube 2G. A generally cylindrical element 56 is xed to the upper end of .the adjacent rod dil; and a pin 58 is disposed through the yoke S4; and a curved bearing 9 mounted in element 5o, to thereby provide relative angular movement between the adjoining control rod sections about a pair of horizontal axes. With the described form of construction, the individual control rod sections 42 are in condition for relative movement into positions of misalignrnent, which may be required because of mechanical variations or tolerances in the components of the rod assembly, because of independent response to differential thermal expension and the like. Furthermore, the described articulated movement of the control rod sections permits the previously referred to angular tilting movement of the entire guide tube Ztl due to lateral ressures exerted there on by virtue of the weight and/or thermal expansion of the adjoining fuel elements.

As previously indicated, the poison material '5d for the individual control rod sections 42 is disposed in spaced relation to the inner walls of the guide tube Ztl. The joints 52 intermediate the control rod sections, however, are in sliding engagement with the inner wall of the guide tube. In this way, there is eliminated any frictional forces otherwise existing between the guide tube Ztl and tube is at the knuckle joints.

The lowerrnost control rod section 4Z (FEGURE l) is detachably connected to the upper end or a push rod 70, through the ball and groove type of latch mechanism seen in FlGURE 1, and this push rod transmits motion from the control rod drive mechanism l2 to the articulated control rod sectons 42. More particularly, the upper end of the push rod t is provided with a plurality of transverse openings 72 in which are seated ball bearings 7d or the like. An outer sleeve 7o on the push rod normally holds the ball bearings 74 in the inner position seen in FIGURE 1, wherein they provide a locking engagement between the push rod 7h and the lower end of the lowermost control rod section 42. In order to release the control rod assembly from the push rod, the push rod tl is elevated, by means to be ldescribed later, until balls le@ move into groove 162, thereby allowing sleeve '76 to drop down and permit the balls '74 to enter an annular groove 78 in the interior of the upper end of the sleeve 76 after further upward movement of rod 7tl. In this condition, the control rod is released for movement upwardly away from the push rod. The guide tube can then be rotated about its axis to tree its bayonet connection 26 with the sperical supporting bearing 2d, and the entire control rod assembly l@ can be moved upwardly through the reactor core in the reactor vessel.

As indicated by the arrows in FGURE l, a portion of the primary coolant for the gas-cooled reactor is utilized to also cool the control rod assembly l0. ln this respect, the gas coolant which flows below the bottom grid plate i8 enters the opening Sil in the grid plate, which is provided to receive the control rod support bearing and the upper end of the push rod housing 32, and then flows through ports or openings 8d through the push rod housing 82 into the interior or" bearing As the coolant Hows upwardly it encounters the knuckle joint $5, which is generally similar to the joints 52 and which connects the push rod 7h with the lowermost control rod section 42, at which joint the gas coolant passes through longitudinal grooves Se formed alongside the internal portion of this joint. At the upper part of th coolant flows into the center or t 'ng rod d2. Shortly after the coolant enters the axial portion of the support rod d2, it divides into two paths, with a portion of the coolant continuing axially and internally or" the support rod and another portion of the coolant flowing through ports 88 into the space provided between the support rod le and the interior wall of the poison section Sil. rlfhus, there is provided two parallel paths through the control rod section for the low of the coolant, namely, the spaces internally and externally of the support rod walls, respectively. As the gas coolant encounters each knuckle joint 52 in turn, it ows through the interior of the joint in the general manner reviously described and then again divides into the parallel paths just mentioned. ln this respect, the coolant which has moved upwardly through the center of one of the support rods ift ilows through ports @il at the `upper end thereof, in order to merge with the coolant flowing around the outside of the Support rod and pursue a course through the interior of the next knuckle joint 52. Further, it will be noted that the nox to the lowermost poison section :Sil is provided 89 at opposite ends thereof. These will, of course, permit coolant to flow externally of the poison section, but these ports are provided primarily for the entry of the gas coolant into the control rod assembly from the passage d@ when the control rod is disposed in its lowered position.

As the coolant reaches the upper end of the interior of the control rod assembly lil, the gas coolant merges inside the up support rod, through the ports 94 in the support i, and then continues its flow upwardly through an axial passageway in the top reflector section 35 to enter the pressure vessel at a position above the control rod.

With the described arrangement, it is s en that there is provided parallel paths of llow internally and externally or" the support rods. With the control rod fully 'nserted as shown in FIGURE l, the major portion of the coolant will flow through the metal support rods 214, with a somewhat esser llow externally of the rods. In the event that there is a reduction in primary coolant llow through the core, the reactor of course would be scramrned immediately.

ln the event that the control rod is partially or entirely withdrawn from its scrammed position, the coolant ows into the ports @da FEGURE 3) in the push rod hous- 1ng 32 and then through the ports 89 (FIGURE l) into the interior of the control rod assembly. ln such case, of course, the major portion of the coolant would flow upwardly and, therefore, those sections of the control rod subjected to the greater amount of heat would receive the greater cooling effect.

As indicated in FEGURE 1, the lower portion of the control rod support bearing housing is preferably llxed integrally with the push rod housing 32 and forms an extension theeof. The push rod housing extends through the bottom wall ld of the pressure vessel (FIG- URE 3), and the lower end of the housing 252 is dispose within a generally cylindrical housing section lil@ which extends through the biological shield lil?. for the reactor.

he preferably solid push rod "tl described above, which is connected at its upper end to the control rod, is in turn connected at its lower end to a hollow push rod (FIGURE 3) which receives the lead screw lilo of a screw actuator through its center bore lill). The hollow push rod ldd and the solid push rod 7f3 are interconnected by a latch mechanism il@ of the ball type previously described, which permits disconnection of the two push rods in response to relative axial movenient therebetween. The hollow push rod is keyed or splined to the housing section 136 by means of the spline indicated at M2 in FIGURE 3. Section i315 is in turn keyed to section lill), to thereby prevent relative rotation between the push rod and its housings and thus restrict the push rod lo@ to axial movement. The lead screw les extends below the hollow push rod ltl'd,

and a ball nut lill is xed in position on the lower end ol tle push rod. Consequently, rotation of the lead screw results in axial movement of the two push rod sections llland 7'@ and of the control rod sections d?. connected thereto. Adjacent the lower portion of the housing section for the screw actuator is a transverse passage or port communicating with a source of pressure gas, such as helium, which maintains the inte rior of the housing 160 at a pressure somewhat greater than that of the coolant in the reactor vessel, to thereby provide for a buffer llow inwardly of the vessel and insure that iission products do not reach the control rod drive. In this respect, it will be understood that suitable passageways are provided for this butler flow, for example, by providing suiiicient tolerance between the push rod and its housing sections.

The control rod drive housing is in two sections, with the upper section comprising the housing 100 referred to above and including the screw actuator and the lower portion of the hollow push rod 104, and a lower housing section 118 containing a hydraulic motor and control valves for regulating ilow between a suitable source of pressure fluid and the motor. The hydraulic motor 1219 in the illustrated embodiment is of the axial piston type, wherein there is provided a body 122 delining a plurality of parallel chambers 124 therein each containing a piston 12e. The motor 120 also includes a central drive shaft 12S supporting a wobble plate 131) which is engageable by the piston 126 to translate the axial movement ot the pistons into rotary movement of the pump drive shaft 12S. Since this form of motor is known and does not constitute an essential portion of the invention, it is believed that a more detailed description thereof is unnecessary.

The upper end of the motor drive shaft 128 is coupled with the lower end of the lead screw 1% by means of a back-stop clutch 132, such as that presently sold under the designation Formsprag Rev-Lock clutch. This form oi clutch permits torque to be applied to the lead screw 1% only from the motor drive shaft 128 and, therefore, prevents any back creeping or reversal of the control rods due to conditions existing in the screw actuator or in the control rod assembly 1t). The backstop clutch 132, as well as a rotary face type diterential pressure seal 134 at the lower end of the lead screw 106, is disposed in the lower end of an actuator support tube 13e, which is slidable in the outer containment or housing 164) but which is selectively axially iixed in position in the housing 100 by means of a vertically adjustable iitting in the form of a support nut 138. This support nut 13S also provides means for releasing the latch mechanism at the upper and lower ends oi the solid push rods 719. More particularly, when the separable hydraulic motor housing 118 is removed, the actuator support tube 136 can be moved axially relative to the support nut 138, through rotation ot the latter, to thereby raise or lower the entire screw actuator assembly and the push rod 1114. Lowering of the actuator assembly and push rod 164 relative to its normal lower stopping point positions latch 1119 so that the balls 1515 (FGURE 3) are moved into the grooves 152 and the balls 154 are then moved into groove 156, to permit hollow push rod 1114 to be separated from the solid push rod 79.

ln order to separate the upper end of the solid push rod 7@ from the control rod proper, the support nut 138 is rotated to eiiect upward movement of the actuator assembly and the lower push rod 104. Referring to FGURE. l, this upward movement of rod 104 causes push rod section '71? to move upwardly beyond its normal uppermost position until the balls 161i are aligned with groove 162 in housing S2, and further upward movement of rod 711 relative to sleeve `"i6 causes balls 74 to move into groove 73, thereby separating the push rod and control rod.

The hydraulic motor drive shaft `12% (.FiGURE 3) also includes thereon a gear 180, which is connected in driving relation to a position transmitter indicated generally at 132 so as to provide information externally of the reactor and remote t erefrom regarding the relative position of the control rod in the vessel. The gear 186` on the motor drive shaft 128 is also connected in driving relation with a deceleration valve screw actuator '184 which is adapted to control the rate of deceleration of the control rod in a manner to be described.

For the operation of the hydraulic motor l there is provided, preferably externally of the motor housing 12, a pressure fluid pump assembly 136 including a reservoir for the pressure fluid. The various control valves intermediate the pump and the hydraulic motor 1219, as -well as the conduits and passages intermediate the pump and the hydraulic motor, are illustrated schematically in FlG- URE 3. ln this ligure, it will be noted that the pump discharge `line includes a manual shut-oil valve 18S as well as a solenoid operated isolating valve 191i.

The isolating valve 19t? is normally held open and is intended to be used to isolate the pump from the motor. Downstream from the isolating valve 1911, the pump discharge iiuid passage branches in two directions, with each branch controlled by a check valve to prevent return flow in the event that the pump 136 fails. One of these branch discharge lines, namely, line 192 leads to a regulating valve 194 which is eliective -to control the direction of flow to and from the hydraulic motor body 122. The regulating valve i1% is also preferably solenoid operated in opposition to a biasing spring. The downstream side o the regulating valve 19e includes two passages 1% and 198, one of which (19S) directs l'luid upwardly to the lett side of the hydraulic motor and the other directs pressure fluid into passage 2li@ and to the right side of the motor.

The regulating valve 19e is selectively movable trom a remote control station to any of three positions to thereby determine the direction oilow into the motor and thus control the direction of rotation of the motor shaft 128. The return ow of pressure fluid entering valve 194 is directed into passage 199* and thence back to the reservoir of the pump system 1%. An adjustable flow control valve Zilli is disposed in the passage 1% and, as will appear more clearly later in the description, this valve is primarily for the purpose o determining the rate of return flow of the lluid from the motor under scram conditions.

Also disposed in the hydraulic circuit for the control rod drive motor 1211, is an accumulator 2tl2 including a piston 2dr-t therein. The accumulator chamber is in fluid communication at its lower end with a suitable source 2% of high pressure fluid, which is preferably a gas, such as helium. The upper portion or the accumulator chamber is in communication through the passage 2% with the hydraulic motor 1121i, when the valve 21@` is open, and is also adapted to receive hydraulic fluid from the pump 1&6 through the branch discharge passage 192:1. The llow oi pressure fluid through the passage 2tlg is controlled by the solenoid operated scram control valve 210, which is normally closed to thereby retain a predetermined pressure in the accumulator chamber and in the passages 19251 and the portion of passage 2% upstream oi the valve, with the piston 2M approximately midway of the length ol the chamber. Under scram conditions, the scram control valve 210 opens and the high fluid pressure in the accumulator charm-ber 2&2 is immediately discharged through passage 238 to operate the motor 1211 at a very high rate of speed and in a direction to effect scram movement of the control rod 113 into the reactor core. Furthermore, the scram signal releases regulating valve 194 .to rnove to a closed position, by action of its biasing spring, blocking passages 1% and portion 19351 of passage 19S. The pressure iiuid discharged by pump 156 is diverted through passage 192e into passage 29S, where it merges with the pressure fluid from the accumulator 2112 to aid in the scram operation of the control rod. Under the described scram conditions for the uid motor system, the return flow from the motor 12? is through the passage 1%, past the scrarn velocity ilow control valve 2119, through a by-pass passage 212, and through the scrarn control valve 210 into return passage 199 to the pump 186i.

In order to control the deceleration oi the control rod 1@ toward the end o its stroke, during scram movement of the control rod, there is provided a deceleration valve 22d which is movable to efiect the return tlow of iluid through the passage Mii. More particularly, this valve is illustrated as being a spring biased valve and having a cam follower or roller 2li projecting from the valve housing in position yfor engagement by a cam at the lower end of the screw actuator As indicated previously, the screw actuator' `ttt-ld is driven from the motor drive shaft 12S, and as the control rod approaches the upper end of its path of travel, the screw actuator 18d has caused its axially extendable screw portion 224i to move downwardly sufficiently for the cam surface 222 to engage the cam follower 221 on the deceleration valve 22d. Continued movement of the control rod is accompanied by continued downward movement of the cam surface Z22, which, in turn, causes the valve 22@ to move inwardly and thereby throttle the return flow of fluid through the passage it is seen, therefore, that the described arrangement provides for operation or" the control rod drive motor at a predetermined regulated speed under the influence or the pump 136 and the regulating control valve 19d, and also provides for acceleration of the control rod movement, during a scram emergency condition, through use of the pressure fluid which is accumulated in chamber 262 and held in readiness for discharge to the drive motor. In addition to the means described herein, it will be understood that various safety precautions should also be provided, including means for monitoring the position of the piston 26d in the accumulator chamber 2tlg to determine its readiness, as well as suitable means for otherwise detecting leakage or" the pressure fluid in the system.

Although shown and described with respect to particular apparatus, it will be apparent that various modifications of the described structure might be used to advantage and without departing from the principles of this invention. For example, in place of the articulated control rod structure described herein, including the plurality of support rods dfi, a single, hollow support rod might extend the length of the control rod within the guide tube 2d. In such modification, the support rod will be provided 'with sufllcient flexibility to accommodate the limited tilting of the guide tube E@ described above.

We claim:

l. h1 a nuclear reactor including a vertically extending pressure vessel, a control rod assembly comprising a tubular guide sleeve mounted in the lower portion of said reactor vessel in an upwardly extending position and in a manner affording limited angular movement of said sleeve about its lower end, a control rod disposed in said guide sleeve and comprising a plurality of elongated neutron absorbing sections interconnected in a manner alfording angular movement therebetween, a push rod releasably connected to the lower end of said control rod and extending downwardly from said vessel in generally axially extending relation to said guide sleeve, control rod drive means connected to the lower end of said push rod, said drive means comprising a rotatable drive shaft, a reversible hydraulic motor connected with one end of said drive shaft, a screw drive means interconnecting the other end of said drive shaft with said push rod in a manner affording axial movement of the latter in response to rotary movement of the former, passage means connecting said llrst -control valve disposed in controlling relation to said rod at a speed considerably greater than said ilrst speed.

2. In a nuclear reactor having a vertically disposed pressure vessel, a control rod assembly comprising a manner aiordof the latter in response to rotary movement of the former, a backstop clutch mechanism connecting said screw drive means with the other end of the motor drive shaft, a source of pressure tluid, passage means connecting said source with said motor, a regulating valve disposed in said passage means and operable to direct pressure fluid therein so as to afford selective rotation of said motor in either direction at a predetermined irst speed, an additional source of accumulated pressure fluid in communication with said passage means, and a scram control valve in position to connect said additional source with said motor in a manner affording selective rotation of said motor -in a direction providing for upward movement of said control rod at a speed considerably greater than said rst speed, so as to scram said rod to its full position within the core of said nuclear reactor.

3. For use with ya control rod in a nuclear reactor, a control rod drive means comprising a fluid operated comprising a connecting said screw actuating means so that applied to the latter only by said drive shaft, means placing said lluid therebetween, a directional control said passage means and operable to sesource and in a selected direction, an accumulator chamber having fluid therein under pressure and having a fluid discharge opening in communication with said passage means, and a scram control valve disposed and operable to control the flow of fluid through said discharge opening.

4. Control rod drive means comprising a fluid operated motor including a rotatable drive shaft, a source of pressure fluid, a first passage means placing said source in communication with said motor to direct the exchange of pressure fluid therebetween, a directional control valve disposed in said first passage means and operable to selectively direct pressure fluid to said motor to effect rotation of said drive shaft in either direction as desired, an accumulator chamber, Ia piston in said accumulator chamber, expandable pressure fluid in said chamber on one side of said piston, a second passage means placing the other piston in uid communication with said directional control valve, a scram control valve in said second passage means tion valve to a predetermined throttling position after the drive shaft has moved a predetermined number of rotations in one direction.

5. Control rod drive means comprising a reversible luid oper-ated motor including ya rotatable drive shaft, a so-urce of pressure fluid, a iirst passage mea-ns placing said source in communication with said motor to direct the exchange of pressure iiuid therebetween, a `directional control valve disposed in said irst passage means and operable to selectively direct the ilow of pressure fluid between said source and said motor so as to effect rotation of said drive shaft in either direction as desired, an accumulator chamber, a piston in said accumulator charnber, means providing expandable pressure fluid in said chamber on one side of said piston, a second passage means placing the other side of said piston in lluid communication with said hydraulic motor and lay-passing said directional control valve, and a scram control valve in said second passage means in position to control the flow of fluid between said vessel and said motor, and means connecting said drive shaft with said deceleration valve in a manner affording movement of said deceleration valve to a predetermined thrott 'ng position in the 'return ow passage from the motor iater the drive shaft has moved a predetermined number of rotations in a predetermined direction.

6. A control rod assembly comprising `a guide tube adapted to be mounted at its lower end Within a nuclear reactor vesselV in generally vertically extending relation thereto, a centr-ol rod disposed in said guide tube and comprising a plurality of elongated sections, joint means interconnecting said elongated sections in a manner affording articulated movement therebetween, said joint means having, a sliding lit with the inte-rior wall of said guide tube and having a passage therethrough affording the flow 'of fluid longitudinally of said guide tube and between opposite sides of said joint means, and each of said sections including an axially disposed hollow support rod, a generally tubular element containing neutron absorbing material and disposed in cio-axial spaced relation to said support rod and said guide tube, and including transverse passages through said support rod at opposite ends thereof, whereby a gas coolant within said guide tube is enabled to llcw in generally parallel paths through said support rod and between said support rod and said tubular element.

References Cited in the file of this patent OTHER REFERENCES Control of Nuclear Reactors and Power Plants, McGraw-Hill, 1955, by Schultz, pages 98-123, particularly FlGS. 5-1, 5-3, 5-4, 5-11, 5-17, and 5-18, 

1. IN A NUCLEAR REACTOR INCLUDING A VERTICALLY EXTENDING PRESSURE VESSEL, A CONTROL ROD ASSEMBLY COMPRISING A TUBULAR GUIDE SL.EEVE MOUNTED IN THE LOWER PORTION OF SAID REACTOR VESSEL IN AN UPWARDLY EXTENDING POSITION AND IN A MANNER AFFORDING LIMITED ANGULAR MOVEMENT OF SAID SLEEVE ABOUT ITS LOWER END, A CONTROL ROD DISPOSED IN SAID GUIDE SLEEVE AND COMPRISING A PLURALITY OF ELONGATED NETURON ABSORBING SECTIONS INTERCONNECTED IN A MANNER AFFORDING ANGULAR MOVEMENT THEREBETWEEN, A PUSH ROD RELEASABLY CONNECTED TO THE LOWER END OF SAID CONTROL ROD AND EXTENDING DOWNWARDLY FROM SAID VESSEL IN GENERALLY AXIALLY EXTENDING RELATION TO SAID GUIDE SLEEVE, CONTROL ROD DRIVE MEANS CONNECTED TO THE LOWER END OF SAID PUSH ROD, SAID DRIVE MEANS COMPRISING A ROTATABLE DRIVE SHAFT, A REVERSIBLE HYDRAULIC MOTOR CONNECTED WITH ONE END OF SAID DRIVE SHAFT, A SCREW DRIVE MEANS INTERCONNECTING THE OTHER END OF SAID DRIVE SHAFT WITH SAID PUSH ROD IN A MANNER AFFORDING AXIAL MOVEMENT OF THE LATTER IN RESPONSE TO ROTARY MOVEMENT OF THE FORMER, A SOURCE OF PRESSURE FLUID, PASSAGE MEANS CONNECTING SAID SOURCE WITH SAID MOTOR, A FIRST CONTROL VALVE DISPOSED IN CONTROLLING RELATION TO SAID PASSAGE MEANS AND OPERABLE TO DIRECT THE PRESSURE FLUID SO AS TO AFFORD SELECTIVE ROTATION OF SAID MOTOR IN EITHER DIRECTION AT A PREDETERMINED FIRST SPEED, A PRESSURE FLUID ACCUMULATING CHAMBER IN FLUID COMMUNICATION WITH SAID PASSAGE MEANS, AND A SECOND CONTROL VALVE DISPOXED IN CONTROLLING RELATION TO THE DISCHARGE SIDE OF SAID ACCUMULATING CHAMBER AND OPERABLE TO REALEASE PRESSURE FLUID FROM SAID CAHMBER INTO SAID PASSAGE MEANS, TO THEREBY OPERATE SAID MOTOR AND EFFECT UPWARD MOVEMENT OF SAID CONTROL ROD AT A SPEED CONSIDERABLY GREATER THAN SAID FIRST SPEED. 